3D NAND

# Multi-Deck String Stacking as the Solution

## 14. 14. Multi Deck String Stacking as the Solution

Rather than pushing one single etch all the way through an ever-taller stack, the industry's answer to the aspect-ratio and bowing problems from the previous two steps was to split the stack into multiple shorter decks, etch and process each deck's channel holes separately while its aspect ratio is still manageable, and then join the decks vertically into one continuous string. A two-deck 128-layer part, for example, might build and etch a 64-layer lower deck completely, then build and etch a separate 64-layer upper deck on top of it, each deck's own channel-hole etch facing only half the aspect ratio a single 128-layer etch would have faced. The channel holes in the upper deck must land precisely on top of the corresponding holes already etched and filled in the lower deck, so that the macaroni channel continues as one electrically continuous tube running through both decks rather than two disconnected tubes stacked on top of each other. This deck-splitting approach is what let the industry keep raising total layer count well past what a single-etch process could reliably achieve, trading one very hard etch for two or more moderately hard etches plus a new problem of its own: getting the decks to line up, which the next step in this series takes up directly.

Splitting One Hard Etch Into Several Easier Ones 14. Multi Deck String Stacking as the Solution SINGLE-ETCH APPROACH One continuous etch through the full layer count Aspect ratio rises with the full stack height MULTI-DECK APPROACH Stack split into two or more shorter decks Each deck etched separately at lower aspect ratio Decks joined afterward into one continuous string two moderate etches instead of one extreme one, same total layer count
Example: a Two-Deck 128-Layer String 14. Multi Deck String Stacking as the Solution LOWER DECK (LAYERS 1-64) Built and etched completely on its own Aspect ratio only half of a single 128-layer etch UPPER DECK (LAYERS 65-128) Built on top of the finished lower deck Its holes must land exactly on the lower deck's holes one electrically continuous channel must still result across both decks

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